X O U or J heads on a 416?

I had X-Heads on my 418 with stock HP exhaust manifolds. Engine dyno peak power of 393 HP @ 4900 RPM and 472 Torque @ 3700 RPM. As stated above; it just couldn't pump any more air and ran out of breath around 5500 RPM. Heads had some bowl work and mildly ported.

Best of 12.96 with 3:55 and 2500 PMC converter on MT Drag Radials.